For warriors (heavy armour, sword and shield/ 2h weapon) they should be able to deflect the fire with a special resistant shield, stun the dragon and create an opening allowing him to finish the dragon by thrusting his sword into the dragons abdomen.
For mages and magic users they should have a arsenal of spells at their disposal to take dragons down in slot of ways depending on what school of magic they specialise in. Destruction mages should be able to go on an all out offensive an use offense as a good defence e.g. stopping the fire of a dragon by shooting back with a frost spell and fighting it with lightning. An illusion Mage could go invisible and suprise it with trickery and a conjuration Mage could use summons to distract it and use others to overwhelm it.
A stealth based character could sneak up on the dragon and be able to identify it's weak spot and go for it sneakily. Or jump on it's back and climb towards it's head shadow of the colossus style. A marksman based character can pepper it with arrows and poisons aiming for weak points like the eyes/ ankles etc.
I think none of this should be possible at the start of the game though. I think in the first 15-20 levels you should walk around a corner see and dragon and think "ohh ****" and run for your life being force to run into a dark mine shaft to escape the dragon. Killing dragons should be a feat hard to accomplish even for a dragonborn. The dragon shouts should be acquired later on in the game and should factor in greatly during dragon fights.